This invention relates to echo cancellation in communication systems.
In typical full duplex data communication, a local data terminal may simultaneously send and receive data to and from a remote data terminal on, e.g., a single two-wire telephone line. Hybrid couplers are located at both ends of the channel for the purpose of isolating the incoming and outgoing signals. Because the hybrid couplers operate imperfectly, the outgoing signal may be partially reflected at the local hybrid coupler in the form of a so called near echo and at the remote hybrid coupler in the form of a so-called far echo. Both echoes corrupt the incoming signal.
The near echo is essentially identical in carrier frequency to the local outgoing signal. It can therefore be eliminated easily by a conventional near echo canceller in the form of an adaptive linear transversal filter that simulates the near echo transfer function which produced the near echo. For this purpose, the complex signal points for the outgoing signal are passed through the near echo canceller to generate a simulated near echo signal which is simply subtracted from the incoming (near-echo corrupted) signal.
Correcting the far echo is not as simple, because it may have a continually shifting phase (phase roll or frequency offset) relative to the original outgoing signal. This may be caused, for example, by small differences (e.g., 1 Hz) between the carrier frequency of the outgoing signal and the carrier frequency of the far echo signal that occur when circuits used to step up and step down the signal carrier frequency (e.g., for a satellite hop) are not perfectly matched. Because of the non-linearity in the far echo transfer function (represented by the changing phase), a far echo canceller in the form of a linear transversal filter is unable to adequately simulate the far echo transfer function.
One known solution is to continually correct the phase of the complex output of a conventional far echo canceller in accordance with a detected difference in phase between the simulated far echo signal and the actual far echo (in the local incoming signal).
In such a scheme, the signal points for the outgoing signal are passed through a conventional bulk delay element that simulates the delay associated with the distance to the remote terminal. The far echo canceller then uses the delayed signal points to generate the complex simulated far echo signal. After phase correction in the complex domain, the real part of the simulated far echo, like the simulated near echo, is subtracted from the incoming signal.
It is also known to cancel an echo of a telephone voice signal by phase correcting complex components derived from the voice signal and providing a real-valued phase corrected signal to an echo canceller, but in that scheme the updating of the phase correction circuitry is inhibited during double talk.
It is known that the power level of the far echo is unpredictable, but is usually lower than the level of the signal from the remote transmitter, and that the frequency of the phase roll is relatively low compared to the signaling rate.